Once the payment is completed, MercadoPago sends a notification to the seller’s website, with information about the order (topic = merchant_order) and the payment (topic = payment). Because of that notification, you can keep your information about the payment process synchronized.

The notification will be sent to the URL that you specified when the Preference was created (&PreferenceSDT.notification_url )

See the ReceivesNotification procedure to know how to handle the received notifications.